1. Field
This application relates to Receivers used in services where the received signals are intermittent.
2. Prior Art
Receivers used in services where the received signals are intermittent have a problem in detecting when the Receiver Chain of equipment (e.g. Antenna, cabling, connectors, Receiver, power, etc.) has an impairment or fault.
In RADAR applications, if a response is not detected to an interrogation signal, is it because there are no targets or is it because the Receiver Chain of equipment is impaired or faulty?
In two-way radio applications, if a response is not received from a called party, is it because the called party is not responding or is it because the Receiver chain is impaired or faulty?
Similarly, in one-way (receive-only) radio applications, if a signal is not received within a pre-specified period, is it because there is no information available or is it because the Receiver Chain of equipment is impaired or faulty?
In PRIMARY or AREA SURVEILLANCE RADAR, use is sometimes made of signals reflecting Permanent Echoes, to verify the operation of the Receiver Chain of equipment; if these Permanent Echoes are available and detectable within the RADAR coverage area. However, if the RADAR uses Moving Target Indication, Moving Target Detection or Doppler Processing filters; these (stationary) Permanent Echoes are eliminated from the received signals.
In Secondary Surveillance Radar and Automatic Dependent Surveillance-Broadcast; use is sometimes made of a Test Transponder, sometimes called a Position Adjustable Range Reference Orientation Transponder or PARROT. This Test Transponder needs to be placed at a distance from the Receiver so it requires real estate and reliable power. It usually also requires reliable communications so it can be monitored by Remote Maintenance Monitoring.
A solution to these problems is to regularly synthesize a Test Signal at a pre-determined interval, inject it into the Receiver, and have a method of verifying that the Test Signal is detected.